Branches of China's biggest banks have suspended financial transactions for North Koreans, employees told AFP, suggesting that Beijing has pursued stronger measures against its nuclear-armed ally than previously thought.

Staff at branches in Beijing and the border city of Yanji -- a major trade and transportation hub between the two neighbours -- said their banks have banned North Koreans from opening new accounts and some have even started to close existing ones.

The restrictions were imposed well before the United Nations Security Council unanimously approved, with China's blessing, new sanctions on Pyongyang on Monday following its latest and largest nuclear test.

Employees at several branches of the country's "big four" -- Industrial and Commercial Bank of China, Agricultural Bank of China, Bank of China and China Construction Bank -- confirmed the financial curbs for North Korean clients.

"We have frozen their accounts, which means they cannot withdraw (money)," a staff member at a Yanji branch of China Construction Bank told AFP.

"They cannot use (their accounts) in Yanji anymore, as well as our services... We have already started to inform them to cancel their account. If they can cancel, we let them cancel. If they cannot, we will not let them use it," the staffer said.

An employee at the Industrial and Commercial Bank of China in Yanji said the restrictions began last year or the previous year.

"We also won't open new accounts now. We offer no service to them. Opening accounts or foreign currency operations, we don't offer such services to them," the employee said.

Other local bank branches said the bans have been carried out for a while, but they did not remember exactly when.

Some said they have received a written document on the ban but others said there has only been a "verbally delivered" message.

A staff member at a Beijing branch of China Construction Bank said they received a notice in May, and North Koreans can no longer conduct transactions.

An Agricultural Bank of China employee in Beijing said North Koreans are barred from opening new accounts but those with current accounts can carry out transactions.

Zhang Liangui, a professor at the Chinese Communist Party Central Committee school, said the ban is "very normal" and in accordance with UN resolutions.